As far as I know the GT has the same engine the MR2 spider has. I`d buy a turbo kit from the spider (there are a couple). I`m sure that you have to make changes on the Intake
and the exhaust due they not located on the same place.
The 1zzfe engine`s 1 to 10 compressio r. seems good for turbocharging. Here you find some kits:http://monkeywrenchracing.com/toyota.html
They`d also help you to give same advises to make the mods.
I was planning to turbo my car they were very friendly. Good luck!